Two low molecular weight infectious RNAs (viroids) will be investigated: one engenders no symptoms, but such an infection can protect hosts from the severe disease caused by the other. The studies anticipated in this proposal are designed to provide fundamental information on the mode of replication, and on the sequence homologies of these viroid RNAs to one another, and to the DNA of their hosts. They will be studied with respect to their: 1) Sequence homology, by fingerprint analysis of the labeled RNAs, and by hybridization with complementary molecules. 2) Relationship of the viroid RNA to host and non-host DNA by molecular hybridization. 3) Possible genesis, as induced by X-ray irradiation of potential host species.